1. Field of the Invention
The present invention relates to a conducting polymer coated electrode of a metal oxide electrochemical pseudocapacitor and a method of manufacturing the same, and more particularly, to a conducting polymer coated electrode of a metal oxide electrochemical pseudocapacitor having an improved quality by coating a conducting polymer having a high electric conductivity and a method of manufacturing the same.
2. Description of the Related Arts
A metal oxide electrochemical pseudocapacitor lies in a field different from any field of a lithium rechargeable battery, an electrical double layer capacitor (EDLC) and a conducting polymer capacitor.
The metal oxide electrochemical pseudocapacitor has similar characteristics as those of a capacitor with the EDLC and the conducting polymer capacitor. That is, electrochemically, a large amount of electric energy is charged/recharged in a rapid time period. Meantime, the metal oxide electrochemical pseudocapacitor has similar characteristics to the lithium rechargeable battery. That is, a metal oxide is used as an active material of an electrode.
The metal oxide electrochemical pseudocapacitor will be described in detail below.
Supercapacitor or ultracapacitor is a kind of the capacitor which can storing tens times to thousands times of electric energy in a unit volume when compared with an aluminum electrolytic capacitor employing the conventional aluminum foil as an electrode. The supercapacitor typically includes three types of an EDLC, a metal oxide electrochemical pseudocapacitor and a conducting polymer capacitor. Technically and commercially, the EDLC attains superiority among them. The EDLC employs an active material of an electrode, having a wide surface area such as active carbon. Electric double layer phenomenon formed at the contacting portion of the surface of the electrode material and an electrolyte becomes the basic source of the energy storage.
The basic energy storing source of the metal oxide electrochemical pseudocapacitor is an electrochemical reaction produced from metal oxide such as a battery. The difference with the lithium rechargeable battery lies in the electrochemical characteristic differences of the general battery and the capacitor. That is, the battery shows a plateau portion in a graph obtained by designating voltage with time, at a constant voltage during charge/discharge. However, the metal oxide electrochemical pseudocapacitor shows a straight line.
The term of xe2x80x9cpseudocapacitorxe2x80x9d comes from the following reason. The characteristic of the capacitor is obtainable from the formation of the electric double layer as in the EDLC or the aluminum electrolytic capacitor and is hard to obtain from the electrochemical reaction. However, some kinds of metal oxides do not show the characteristics of the battery but the characteristics of the capacitor. That is, the reaction of the metal oxide electrochemical pseudocapacitor concerning the energy storage is similar to that of the battery, however, the characteristics of the metal oxide pseudocapacitor are similar to those of the capacitor. From this point of view, the term of xe2x80x9cpseudoxe2x80x9d is applied for the meaning of imitation or similarity.
The metal oxide electrochemical pseudocapacitor and the lithium rechargeable capacitor will be compared as follows. They are similar to each other in using metal oxide as the electrode and in using the electrochemical reaction as the energy storage means. However, they show quite different electrochemical characteristics when considering voltage-current-time parameters. For example, the lithium rechargeable capacitor illustrates various peaks in a current-voltage curve, however, the metal oxide electrochemical pseudocapacitor show a shape similar to a tetragon in the same curve.
Now, the metal oxide electrochemical pseudocapacitor and the EDLC or the conducting polymer capacitor will be compared as follows. The electrochemical characteristics of the metal oxide electrochemical pseudocapacitor and the EDLC are similar. In addition, the current-voltage characteristic curve and the voltage-time curve according to the charge/discharge are the same. However, the reaction mechanisms concerning the energy storage are quite different. Also, they use different active materials as an active carbon and metal oxide. The electrochemical characteristics of the conducting polymer capacitor and the metal oxide electrochemical pseudocapacitor also are similar. However, different from the metal oxide electrochemical pseudocapacitor, the conducting polymer capacitor uses the conducting polymer as the electrode.
Generally, the metal oxide pseudocapacitor uses metal oxide as the active material of the electrode. The active material is required to have a high electric conductivity. When the electric conductivity is low, the equivalent series resistance (ESR) of the capacitor increases and the performance at a high output power is deteriorated. In addition, the degree of the practical usage of the active material is lowered and the energy density is decreased. Accordingly, in general, carbon is mixed as a conductive agent. The electrode is manufactured by mixing the active material and carbon as the basic material along with a solvent such as alcohol, water, etc. to obtain a slurry and then by coating the slurry on a current collector.
However, according to the conventional method of manufacturing the electrode, the conducting carbon is mixed. In this case, the carbon should keep a continuous contact from the current collector to each portion of the electrode without a break in order to exhibit a good effect. The continuousness depends on the mixing method and the degree of dispersion of both of the active material of metal oxide and the conducting carbon. The maintenance of the degree of dispersion is very difficult.
The electrochemical reaction which can accomplish the characteristics of the pseudocapacitor occurs at a place where three kinds of reactants meet. The three reactants are a working ion present in an electrolyte, an electron participating in the electrochemical reaction and a surface of the active material which plays the role of a catalyst. The place satisfying this condition is called as an active site. From the view point of forming the active site, the degree of the dispersion of the carbon and the active material is in a close relation with the performance of the capacitor. When mixing the active material and the conducting carbon in order to manufacture the electrode of the capacitor, the degree of the dispersion of the above two materials depends on the mixing condition. Ideally, carbon should make close contact with the current collector in a wide area and the contact between them should be continuous in a chain form without a break. In addition, the carbon chain should make contact with the surface of the active material through an area as wide as possible.
In the electrode manufactured by the above-described method, carbon functions connecting the flow of the electrons initiated from the current collector to the active material so that the electrons participate in the electrochemical reaction occurring at the active material. Accordingly, when the degree of the dispersion of the active material and carbon is not good, the active site decreases to decrease the energy density. In addition, the resistance of the capacitor increases due to the defect of connection between carbon molecules to deteriorate the characteristics of the capacitor at a high output power.
Even when the connecting chain between carbon is good in the electrode owing to the high degree of the dispersion, the resistance generated through a passage of electrons along a grain boundary which is a boundary formed between carbon particles forming the carbon connecting chain, cannot be completely eliminated. Therefore, a method for decreasing the dependency of the electrode performance on the degree of the dispersion and the resistance due to the carbon chain is required.
Meantime, a lithium rechargeable battery using an electrode coated with conducting polymer has been reported as follows. See F. Coustier, D. B. Le, S. Passerini and W. H. Smyrl, xe2x80x9cHigh Surface Area V2O5 as Host Material for Lithium Intercalationxe2x80x9d, in the proceedings of the Symposium on Electrode Materials and Processes for Energy Conversion and Storage IV, J. McBreen, S. Mukerjee and S. Srinivasan, Editors, The Electrochemical Society Proceedings Series, PV97-13, Pennington, N.J. (1997). However, an application of the metal oxide electrochemical pseudocapacitor different from the principle of the lithium rechargeable capacitor has not been reported.
It is an object of the present invention considering the above-described problems, to provide an electrode having an increased specific capacitance by coating conducting polymer having a high electric conductivity onto the conventional metal oxide electrode.
Another object of the present invention is to provide a capacitor having an increased storage capacity of an electric energy and an improved performance at a high output power by reducing an equivalent series resistance.
Another object of the present invention is to provide an advantageous method of manufacturing an electrode described above.
To accomplish the object, there is provided in the present invention an electrode of a metal oxide electrochemical pseudocapacitor comprising a current collector and an active material coated on the current collector. The active material includes metal oxide and is coated with a conducting polymer on a surface thereof.
Particularly, the conducting polymer is at least one selected from the group consisting of polypyrrole, polyaniline and polythiophene.
In addition, the current collector preferably includes the conductive material coated on a surface thereof. More preferably, carbon is used as the conductive material. Further, the active material includes manganese dioxide as a main component and more preferably, includes amorphous manganese dioxide as a main component.
Another object of the present invention can be accomplished by a metal oxide electrochemical pseudocapacitor comprising a plurality of electrodes, an electrolyte and a separator positioned between the electrodes to prevent a contact between said electrodes. The electrode comprises a current collector and an active material coated on said current collector. The active material includes metal oxide and is coated with a conducting polymer on a surface thereof.
As for the electrolyte, at least one selected from the group consisting of potassium chloride (KCl), potassium sulfate (K2SO4), lithium chloride (LiCl), lithium sulfate (Li2SO4), sodium chloride (NaCl) and sodium sulfate (Na2SO4) is preferably used.
Further another object of the present invention is accomplished by a method of manufacturing an electrode of a metal oxide electrochemical pseudocapacitor comprising the following steps. First, an active material is coated on a current collector. A mixture is prepared by mixing a monomer of a conducting polymer and a solvent and an additive is added into thus prepared mixture. Then, the current collector coated with the active material is dipped into thus obtained mixture. After that, the monomer is polymerized to coat the conducting polymer on the current collector coated with the active material.
As for the solvent, at least one selected from the group consisting of distilled water, PC (propylene carbonate) and ACN (acetonitrile) is preferably used. As for the additive, at least one selected from the group consisting of Sodium salt of para-toluenesulfonic acid and TEATFB (tetraethyl ammonium tetrafluoro borate, (CH3CH2) 4NBF4) can be used.
The polymerization can be implemented through an electrochemical reaction. At this time, the polymerization is implemented by keeping a current density of about 0.1-10 mA/cm2 for about one minute to 24 hours. Otherwise, the polymerization can be implemented by keeping a voltage of about 0.5-1.0V for about one minute to 24 hours.
The polymerization also can be implemented by adding an oxidizing agent. At this time, ammonium sulfate is preferably used.
To accomplish further another object, there also is provided in the present invention a method of manufacturing an electrode of a metal oxide electrochemical pseudocapacitor comprising the following steps.
First, a first solution in which an active material is dispersed, is prepared. A second solution in which a monomer of a conducting polymer, a solvent and an additive are mixed, is prepared. Then, the first and second solution are mixed and stirred. An oxidizing agent is added into thus obtained mixture to polymerize the monomer and to coat the conducting polymer on the active material. Then, thus obtained active material coated with the conducting polymer is coated onto a current collector.
According to the present invention, a conducting polymer is coated on the surface of the electrode or metal oxide powder of the conventional metal oxide electrochemical pseudocapacitor to decrease ESR which is an important characteristic of a capacitor and improve high power performance. In addition, the active site where the electrochemical reaction occurs is increased to increase energy density.